(1) Field of the Invention
The present invention relates to a process for developing an electrostatic latent image in an electrophotographic copying machine or the like. More particularly, the present invention relates to a developing process in which an excellent image can be formed while carrying out detection and control of the toner concentration effectively.
(2) Description of the Related Art
In an electrophotographic copying machine, an electrostatic latent image is formed by carrying out electric charging and imagewise light exposure on a photosensitive drum, a magnetic brush of a two-component type developer comprising a toner and a magnetic carrier is formed on a developing sleeve, and the magnetic brush is brought in sliding contact with the photosensitive drum to effect visualization (development) of the electrostatic latent image.
The magnetic carrier used for the two-component type developer is divided into a type having a relatively small value of the specific resistance, such as iron powder or lowly resistant ferrite, and a type having a relatively large value of the specific resistance of high-resistance ferrite etc. The former type has a good reproducibility for the solid portion but is poor in the reproducibility of fine lines, while the latter type has a good reproducibility for fine lines but the image density in the solid portion is poor. Thus, each type has merits and demerits.
Japanese Unexamined Patent Publication No. 60-87373 proposes a development system for obtaining an image having a sufficient resolving power, a sufficient reproducibility of letters and a sufficient density by using a carrier having a specific insulation resistance higher than 10.sup.12 .OMEGA.-cm, in which the ratio of the distance between a developing roll and a regulating plate to the distance between the developing roll and a photosensitive drum is adjusted to from 0.85 to 1.05 and a carrier having a saturation magnetization of 5 to 40 emu/g is used.
Recently, a copying machine is often used not only in a well-air-conditioned office but also in a place in which environmental changes are violent, for example, a factory. Accordingly, a development system capable of providing a stable image irrespectively of environmental changes is desired, and for example, environmental changes are coped with by a combination of a toner concentration sensor and a temperature-humidity sensor.
However, if a carrier having a low magnetic force, for example, a carrier having a saturation magnetization of 5 to 40 emu/g, as proposed in the above-mentioned prior art, is used, the sensitivity of the sensor is reduced and it becomes impossible to control the toner concentration. Alternatively, even if the control is possible, the control accuracy is reduced and various troubles take place.